Mayor Bill de Blasio and City Council Speaker Melissa Mark-Viverito meet with first responders at the site of Wednesday's explosion and buildings collapse in East Harlem.Credit: Marcus Santos-Pool/Getty Images

As firefighters continued to sift through debris in the smoldering wreckage where two East Harlem buildings were leveled, Mayor Bill de Blasio returned to the site of the fatal explosion.

The mayor and other city officials were briefed Thursday morning on first responders' ongoing recovery efforts several feet from where the Park Avenue buildings collapsed, according to pool reports.

The search was described to de Blasio and City Council Speaker Melissa Mark-Viverito, whose district included the destroyed buildings. About 90 minutes before they arrived, a victim was found.

At least seven people were confirmed dead as of Thursday night after four bodies were discovered in the ruins overnight following the blast. A number of people are still missing.

"I can only imagine knowing that at any moment you might find a body, how difficult that is," de Blasio told firefighters at the scene, according to a pool report.

Rescue operations were hindered by a sinkhole that formed after a water main break. Officials said it was too soon to say whether the main break occurred before or after a gas leak, believed to be the cause of the explosion.

De Blasio said later that Thursday's weather also hampered the search.

"They've been fighting through the cold, they've been fighting through the wind -- exceedingly difficult circumstances -- and they have stuck with it," he said.

De Blasio said the rescue "will continue for an open-ended period of time."

As the mayor left the site in the morning, a pool reporter overheard de Blasio say to to Mark-Viverito, "It is remarkable what they do."